Invitrogen™ Elongase™ is a mixture of Taq DNA Polymerase (Recombinant) and Pyrococcus sp. GB-D thermostable DNA polymerase. The ratio of the two thermostable polymerases is such that the processivity and activity of Taq and the 3′-5′ exonuclease of the Pyrococcus sp. GB-D polymerase allow for the amplification of fragments up to 30 kb (up to 30 kb with lambda sequence and up to 20 kb for a single copy sequence in genomic DNA) and the greater amplification of smaller fragments. Amplification using Taq DNA polymerase is generally limited to DNA template less than 5 kb. This limitation is believed to be largely due to the inability of Taq DNA Polymerase, which lacks proofreading 3′-5′ exonuclease activity to correct nucleotide misincorporations and continue primer elongation. Including a small amount of proofreading enzyme edits the nascent strand to allow subsequent polymerization by Taq.